Semiconductor chips have found widespread use in a wide range of technical applications. However, they are still very complex and expensive to fabricate. Although silicon substrates can be thinned to very low layer thicknesses, so that they become flexible, these methods are expensive, with the result that flexible or curved microchips are only suitable for applications in which high costs are acceptable. The use of organic semiconductors offers the possibility of inexpensive fabrication of microelectronic semiconductor circuits on flexible substrates. One example of an application is a thin film with integrated control elements for liquid crystal screens. A further application is in transponder technology, in which for example information about a product is stored on what are known as tags.
Organic semiconductors are readily accessible and can be patterned very easily, for example, using printing processes. However, the use of organic semiconductors of this type is currently limited by the low mobility of the charge carriers in the organic polymer semiconductors. This mobility is currently at most 1 to 2 cm2/Vs. The maximum operating frequency of transistors, and therefore of the electronic circuit, is limited by the mobility of the charge carriers. Although mobilities on the order of magnitude of 10−1 cm2/Vs are sufficient for driver applications in the production of TFT active matrix displays, organic semiconductors have not hitherto proven suitable for radiofrequency applications. For technical reasons, wireless transmission of information (RF ID systems) can only take place above a certain minimum frequency. In systems which draw their energy directly from the electromagnetic alternating field and therefore also do not generate any voltage of their own, carrier frequencies of 125 kHz or 13.56 MHz are in widespread use. Systems of this type are used, for example, for identifying or labelling articles in smartcards, identification tags, or electronic postage stamps.
Processes in which semiconducting molecules, for example, pentacene or oligothiophenes, can be deposited as far as possible in an ordered manner have been developed for improving the charge carrier transport in organic semiconductors. This is possible, for example, by vacuum sublimation. Suitable deposition of the organic semiconductor leads to an increase in the crystallinity of the semiconductor material. The improved π—π overlap between the molecules or the side chains allows the energy barrier for charge carrier transport to be reduced. By substituting the semiconducting molecular units by bulky groups during the deposition of the organic semiconductor from the liquid or gas phase, it is possible to produce domains which have liquid crystal properties. Furthermore, synthesis methods in which as high a regioregularity as possible is achieved in the polymer by the use of asymmetric monomers have been developed.
The electrical conductivity of many organic semiconductor materials, as with inorganic semiconductors, can be increased by the introduction of suitable dopants. However, there are problems with achieving positional selectivity during the doping. In the organic semiconductors, the dopants are not tied to a specific position and can move freely within the material. Even if the doping process can originally be restricted to a defined region, for example, the regions around the source and drain contacts, the dopants subsequently migrate through the entire semiconductor layer under the influence of the electric field, which is applied between the source and drain contacts in order to operate the transistor.
It is known that electrically semiconducting polymers are used, for example, in field effect transistors or electronic components which are based on a field effect.
For organic polymers to be used in field effect transistors or similar electronic components, it is necessary for the polymer to behave like an insulator when no electric field is applied, while it forms semiconductor properties or a conduction channel under the influence of an electric field. For example, polyphenylenes or naphthalene derivatives have such properties. However, owing to their insolubility, these are not processable, i.e., these compounds cannot be used to produce field effect transistors.
A semiconductor device having a semiconductor path made from an organic semiconductor material, which is relatively easy to produce with improved electrical properties, in particular, electrical conductivity, is desirable.